Controlled engine exhaust gas recirculation is a commonly used technique for reducing oxides of nitrogen in products of combustion that are exhausted from an internal combustion engine to atmosphere. A typical EGR system comprises an EGR valve that is controlled in accordance with engine operating conditions to regulate the amount of engine exhaust gas that is recirculated to the induction fuel-air flow entering the engine for combustion so as to limit the combustion temperature and hence reduce the formation of oxides of nitrogen.
Since they are typically engine-mounted, EGR valves are subject to a harsh operating environment that includes wide temperature extremes and vibrations. Exhaust emission requirements impose more stringent demands for improved control of such valves. Use of an electric actuator is one means for obtaining improved control, but in order to be commercially successful, such an actuator must be able to operate properly in such extreme environments for an extended period of usage. Moreover, in mass-production automotive vehicle applications, component cost-effectiveness is also essential. An EGR valve electric actuator that possesses more accurate and quicker response results in improved drivability and fuel economy for a vehicle having an internal combustion engine that is equipped with an EGR system. It also provides better control over tailpipe emissions.
One such EGR system is disclosed in U.S. Pat. No. 5,947,092, issued Sep. 7, 1999 and assigned to the same assignee as the present application. That patent discloses a valve head and valve seat that interact to regulate the recirculation of exhaust gas. The valve head is biased into a closed position in contact with the seat by a helical coil spring acting on a locating member crimped on a stem connected to the valve head. An armature subjected to a magnetic field from a coil acts against the spring to open the valve. A surface of the armature contacts a domed surface of the locating member to transmit valve opening forces. A spring-loaded sensor mounted in a housing of the EGR valve biases the armature against the locating member.
Another arrangement of an EGR system is disclosed in U.S. Pat. No. 5,960,776, issued Oct. 5, 1999 and assigned to the same assignee as the present application. In that arrangement, a replaceable plug mounted to the armature contacts, and is free to slide on, a crowned surface of the valve operating rod. The replaceable plug is used as a calibrating device for the EGR valve, to achieve a desired valve opening at a specified coil energization level.
Details of another known EGR valve arrangement 100 are shown in FIG. 1. An EGR valve having a valve arrangement similar to that of valve arrangement 100 is disclosed in U.S. Pat. No. 5,911,401, issued Jun. 15, 1999, assigned to the same assignee as the present application.
A pintle 136 operates valve elements (not shown) mounted in an enclosure (not shown) for regulating the flow of exhaust gas into the intake manifold. Motion of the pintle 136 is restrained by a bearing guide 102 mounted in the enclosure. A disc-shaped shim 180 is mounted on and abuts a shoulder of the pintle 136. A calibration nut 130 is threaded on a threaded end of the pintle 136, and a lower end of the calibration nut engages the shim 180.
An armature 112 applies an opening force on the pintle 136 in response to a magnetic field applied by an electromagnetic coil 160. A stator 195 provides a magnetic path for the field produced by the coil 160. Between the stator 195 and the armature 112 is a non-magnetic sleeve 190. The sleeve 190 restrains and guides the armature 112 as it moves in response to the magnetic field. The armature 112 and the pintle 136 are therefore independently guided.
The armature 112 has a wall 110 with a central hole 111 through which a shoulder of the calibration nut passes. The hole 111 is large enough to permit movement of the armature relative to the calibration nut, thereby allowing independent movement of the armature with respect to the pintle. The calibration nut 130 compresses a wave spring 140 that biases the armature 112 against a surface of the shim 180. The joint between the armature 112 and the pintle 136 therefore allows the armature 112 to float and find its own center in the sleeve 190, while fixing the armature and pintle in an axial direction to stop movement from applied vibration to the valve.
A main spring 150 mounted in a cup-shaped end 170 of the sleeve 190 biases the pintle to return the valve in a closed position when power is removed from the coil 160. The main spring acts directly on the armature 112. The axial position of the armature with respect to the closed position of the pintle is set by varying the overall length of the shim 180. The position is measured for feedback control by a sensor 120.
As the valve is opened by the pintle 136, the main spring 150 is compressed and the force applied by the main spring increases according to the spring constant. That force acts on the armature 112. The calibration spring 140 must therefore be strong enough to counteract the force of the main spring to maintain contact between the armature and the shim.
Recent valve designs have required higher valve closing forces to be exerted by the main spring. Those forces must be counteracted by higher forces from the calibration spring to maintain the armature in contact with the shim. The resulting increased reaction force between the armature and the shim increases frictional forces and reduces the ability of the armature to float with respect to the pintle, resulting in increased valve hysteresis.
There is therefore presently a need to provide an electronic EGR valve that tolerates a high valve closing force exerted by the main spring 150, while having reduced hysteresis during a valve actuation cycle. Such an EGR valve should also minimize manufacturing costs. To the inventors' knowledge, no such injector is currently available.